This invention is related to a new design of and manufacturing method for crash sensors. In patent application Ser. No. 480,273 mentioned above, it is disclosed that a crash sensor can be constructed with a configuration of a square or rectangular flapper swinging inside a closed passage. It is also indicated that such sensors can be made of plastic by a molding process. This present invention provides further improvements on the previous design.
Current crash sensors can be classified into several categories: spring-mass, crush switch, electronic, and gas-damped. This invention is mainly in the last group although it is also applicable to spring mass sensors. A sensing mass in the shape of a flapper is disclosed in the aforementioned patent application. The flapper is coupled with and arranged to move in a housing. The flapper is biased by a spring or magnet toward a first position in the housing. When the sensor is installed on an appropriate location on a vehicle and a crash occurs, the flapper moves toward a second position in the housing. If the crash pulse is of enough magnitude and duration, an electrical circuit is then closed to initiate the protection apparatus associated with the sensing device. During the motion of the flapper, gas is forced to flow through the gap between the flapper and the housing. Due to the sharp edge of the flapper and the small surface area involved in the flow path, the viscous effect is negligible and the gas flow is of inertial type. The characteristic of an inertial flow through an orifice is that the flow velocity is proportional to the square root of the pressure difference across the orifice. This can be alternatively stated that the damping force on the mass is proportional to its velocity squared. This is in contrast to viscosity limited flow where the damping in proportional to the first power of the mass velocity.
One advantage of using inertial gas flow in a sensor is that the gas flow rate is not viscosity dependent. Since gas viscosity is sensitive to temperature variations, the performance of sensors utilizing viscous type of gas flow is significantly influenced by temperature changes. In inertially-damped sensors, the gas flow rate is a function of the pressure difference across the orifice and gas density only. As long as the gas density inside a sensor is kept constant, the behavior of the sensor is much less sensitive to temperature variations than viscously-damped sensors. In order to maintain a constant gas density inside the sensor, the interior of a sensor must be isolated or "hermetically" sealed from the ambient environment. The construction of this invention adapts a design that allows the manufacture of hermetically-sealed crash sensors. A "hermetically-sealed" sensor is defined here as a sensor, which has no openings to the atmosphere and only allows a negligible amount of gas to enter into or escape from its interior over a considerable period. For example, if a sensor is made of plastic and sealed from the atmosphere, the only leakage that can occur is gas diffusion through the plastic material or along the seams or metal to plastic joints.
The configuration of some of the sensors disclosed in the patent application Ser. No. 480,273 consists of a rectangular flapper and a rectangular housing. A flapper, which is the sensing mass for sensing the acceleration of the crash, is a planer member having a thickness in the sensing direction which is much less than its width or height and is arranged to rotate relative to the housing. The flapper is coupled with the housing by a thin hinge on the edge of the flapper, by a knife edge support or other means. The axis of the housing is parallel to or aligned with the desired crash detecting direction. For example, if the sensor is to be used for frontal impact sensing, the sensor should be installed to have the axis of the housing approximately parallel to the front-rear direction of the vehicle. The flapper is arranged to rotate along an axis perpendicular to the axis of the housing.
The parts of the sensor of this invention can be manufactured by the plastic injection molding processes, in which the flapper, the hinge, and the housing are formed in a single piece. If the hinge in made form the same plastic as used for the flapper and the housing, then they can be formed in a single molding process. On the other hand, if the hinge is made from another material, such as metal or a different plastic, it can be formed into its shape first and then insert molded with the plastic parts. The contacts comprising the switching circuit for the sensor are also to be insert molded into the housing. To ensure that the sensor is hermetically sealed, the metallic parts can be first coated by a bonding material which adheres to both the contacts and the plastic. It is known that the contacts and the plastic have different thermal expansion coefficients and thus, if they are not treated, they could separate when the temperature changes and result in leaks. One coating material which is resilient and adhesive and prevents the separation of the metal and plastic materials within the normal crash sensor operating temperature range, usually specified at -40.degree. F. to 250.degree. F., is disclosed in U.S. Pat. No. 3,522,575 of Watson et al and is new to the field of crash sensors. The coating material mentioned in the Watson patent is a phenolic resin with 6 percent content of polyvinyl chloride. This manufacturing method not only eliminates the need of additional assembly steps, but also provides the hermetical sealing for the sensors.
The sealing of the sensor from the ambient environment is important to keep the gas density constant inside the sensor which renders the sensor insensitive to temperature changes. Furthermore, it can also isolate the interior of the sensor from dust and moisture which could interfere with the motion of the flapper or the flow of the gas through the orifice. With this new technique, therefore, the sensor can be assured of a long and reliable life which is very important for automotive safety system components.